Robert Ballard, the National Geographic explorer who found the Titanic, has embarked on a ten-year mission to explore a hidden America—the part of the country that lies beneath the ocean waves.

Underwater America is larger than most people realize. In 1983 Ronald Reagan expanded the country's sovereign rights 200 nautical miles from its shores "for the purpose of exploring, exploiting, conserving, and managing natural resources." In effect, Reagan had doubled the area within the United States' boundaries.

National Geographic spoke with Ballard about why he took on this ambitious project, what's been discovered so far, and where he hopes to go next.

How did you get involved?

When I learned that Jefferson doubled the size of the United States and then Ronald Reagan doubled it again, that caught my fancy. But Reagan didn't follow up as Jefferson did with [Lewis and Clark's] Corps of Discovery. I got involved when I started to realize how huge an area it was.

I have the luxury of having my own trust and having my own ship. I'm a 13th-generation American. I've got exploration in my genes. It just seemed like something that ought to be done. If the government wasn't going to do it, then the heck with it, I was going to go do it.

It was a super-secret sonar system during the Cold War. I won't tell you why they needed it because it's classified. The United States during the Cold War developed—with a company called General Instruments—a multibeam mapping system that was strapped on the bottom of Liberty Ships because they're so big and stable. These Liberty Ships then spent a huge amount of time making maps, and no one knew about it.

Maps generated by sonar have been important for Ballard and his team.

Courtesy Ocean Exploration Trust

When I was in the Navy, I was in naval intelligence and I learned about it. I worked hard to get them to let us use some of these maps, which they did. We used them in the artwork for my very first article for National Geographic magazine, "Dive into the Great Rift," in May 1975. That was the first time the Navy released these maps to the general public. Then we got them to let a company build an unclassified version. It wasn't as good as the military version. Now they finally have declassified the best version.

I've been after this sonar forever.

How does the sonar make maps?

Think of it as a lawnmower. It's a big, monstrous, nine-ton sonar that's attached to bottom of the ship. It sends out a signal like a fan, and then the signal hits the [ocean] bottom and it comes back up. It can digitize and produce a topographic map in real time at full speed, which for us is 12 knots, and the area that the lawn mower is cutting is seven kilometers wide.

You see everything in great detail. It's like someone turns on the light and you can see the mountain ranges, you can see the volcanoes and the calderas, you can see river patterns.

Sonar photos, such as this one of a shipwreck, have been crucial in Ballard's work.

Courtesy Ocean Exploration Trust

What have you found so far?

Wild stuff. Imagine a bowl on the bottom of the ocean that's a mile across, and it's a bowl of salt brine. It looks like a swimming pool, but you're down 5,000 feet. These salt pools are caused by fingers of salt coming up from thousands of feet under the ocean. As the brine dilutes and goes down the side of the bowl, there's billions of mussels living off of it.

Is there anything you're particularly excited about exploring?

Oh, everything. I think a scientist is not supposed to have a vested interest in the answer. They just want to know what it is. I'm an equal opportunity explorer. I don't care whether it's biological, archaeological, geological.

There has to be new things because we've seen so little [of the ocean] and we've discovered a lot in that little. How can you think you were so lucky that you looked at just the right places?

Your ship, the Nautilus, was in the Gulf of Mexico over the summer.

We went out to the BP oil spill site with Ecogig, a consortium of universities looking at the impact of the Deepwater Horizon, particularly on the deep sea corals. Then we also worked with another group from Texas State, and we found three ships, one of them armed and one of them full of stuff. We're not sure if the armed ship is a pirate ship or a ship protecting the two others.

Now we're off Puerto Rico looking at where there might be giant landslides that create tsunamis. That's ongoing this very minute.

We see in the geologic record that giant landslides have taken place off Puerto Rico and off the Bahamas. It's sort of like an avalanche on a mountain in the Alps. When you have these big landslides, they can trigger tsunamis.

The geohazards section of the USGS is exploring with us this area off of Puerto Rico where they think there have been big, big landslides. They're looking back at the history of them to see if there's a rhythm—kind of like your heart beats so often and [the Old Faithful geyser in] Yellowstone erupts so often. What's the cyclicity of this? Do we need to be worried or not worried?

Do you know where you will go next year?

We plan on working in the Gulf of Mexico this year, next year, and a piece of '15. Then in the latter part of '15 we will be off of the California continental borderland from Mexico to Oregon. Then we'll jump across to Hawaii, then work our way through Christmas Island, Baker Island, Guam, and Wake and probably set up shop in Guam.

You've made some significant discoveries in your career. Are there any that really stick with you?

The real discoveries, like the hydrothermal vents and the black smokers. The Titanic was just missing in action. It wasn't a discovery; it was a relocation error. What's really fun are the new ones when they just defy the textbooks like the hydrothermal vents and the giant tube worms and finding black smokers, which explain the chemistry of the world's oceans.

People say, What's your next discovery? I say, I don't think you understand the process. I don't know what I'm going to find, but I'm going to go look.